I had the same problem with my 15 Pro.....for weeks! I finally got the thread to stop breaking by using a 40 weight thread on top and bottom line (both from Superior Threads) in the bobbin. The top thread pulled hard through the tension disk but was eye lashing on the bottom. I have my bobbin set much looser than I have seen recommended. The top thread still pulls pretty hard but my tension is finally right. To stop the thread shredding I went to Organ size 16 needles, my machine hated the needles sent with the machine.
Good luck, I know it is frustrating but keep at it, you will find what your machine likes.
Marilyn